Implants of this type have been in clinical use for approximately 10 years. In contrast to cementable implants, the fixation of the presently stated implants occurs without cement, since the trabecular bones grow into the three-dimensional structure and ossify there. The three-dimensional surface structure of such implants is thoroughly grown through with bone material which, in principle, results in excellent experiences with long-term fixations. Implants of this type are known, for example, from DE-A1-29 10 627, DE-A1-31 06 917 (U.S. Pat. No. 4,718,721) and more recently from DE-U1-90 11 363.
Even when the long-term fixation--as already mentioned--in principle fulfills the desired expectations, the known implants have, nevertheless, the disadvantage that they only form a functional unit with the surrounding bone material in the border areas, in the sense that on account of the different bone thickness structure in areas of the implant, different elastic moduluses in the natural spongiosa of the bone stand in contrast to the uniform surface structure of the implant. In certain areas of the bone spongiosa of a tubular bone, the spongiosa has a rather large cell and pore structure towards the joint ends, but there are areas where the cell and pore size of the bone spongiosa is approximately one to three times smaller than in the first-mentioned area. As an example of such a tubular bone, the human femur can be cited.
It is clear that the trabecular bones, as an integral part of the natural spongiosa, cannot grow well into a homogenous cell and pore structure in the uniform surface structure of the implant. Ultimately, this leads to a prolongation of the healing phase and the reconstruction phase of the bony layers into the implant, and to a shortening of the long-term fixation, because of the fact that the so-called relatives of a trabecular bone group grow into the surface structure, which are not of the group to be attached to a certain location of the spongiosa with a predominating cell and pore structure.
In view of this background, it is the object of the present invention to create an implant with an open-mesh three-dimensional structure at least partly covering its surface, whose healing ability, as well as whose long-term fixation properties, are a clear improvement over the known implants.